The present invention relates to an improved method for the manufacture of aircraft engine cylinder barrels to prevent their premature failure due to hairline cracks or specks thought to be caused by caustic stress corrosion cracking during black oxide treatment.
Stress corrosion cracking, a serious problem in many industries because it may result in a brittle fracture of a normally ductile metal, is a progressive type of fracture, somewhat similar to fatigue. It is due to the combined action of corrosion and tensile stresses. These stresses may be either applied (external) or residual (internal). Cracks, which may be either transgranular or intergranular, depending on the metal and the corroding agent, grow gradually over a period of time until a critical size is reached at which point the stress concentration may cause a sudden brittle fracture of the remaining metal. As is normal in all brittle fractures, the cracks are perpendicular to the tensile stress. Usually there is little or no obvious visual evidence of corrosion.
Nearly all metals are susceptible to stress-corrosion cracking in the presence of tensile stresses in specific environments. For example, austenitic stainless steels, such as those of the 200 and 300 series, are subject to stress-corrosion cracking from chlorides and other halides when under tensile stress. Carbon and alloy steels are susceptible to stress corrosion cracking when exposed to caustic conditions; this phenomenon, often referred to as xe2x80x9ccaustic embrittlement,xe2x80x9d occurs, for example, following exposure to sodium hydroxide solutions; calcium, ammonium, and sodium nitrate solutions; mixed acids like sulfuric-nitric acid; hydrogen cyanide solutions; acidic hydrogen sulfide solutions; moist hydrogen sulfide gas; seawater; and molten sodium-lead alloys. Stainless steels are subject to caustic stress corrosion cracking (CSCC) following exposure to acid chloride solutions, such as magnesium chloride and barium chloride; sodium chloride-hydrogen peroxide solutions; hydrogen sulfide; sodium hydroxide-hydrogen sulfide solutions; and condensing steam from chloride waters. While most metals and alloys, including carbon steel, handle caustic corrosive environments well at room temperature, susceptibility to corrosion and to CSCC increases with increased alloy content, caustic concentration, temperature, and stress level.
Although specks or hairline cracks are known to be a cause of premature failure of engine cylinder barrels, the underlying cause of those specks or hairline cracks has been the subject of dispute among investigators. Investigators have detected specks or hairline cracks in the engine cylinder barrels of downed aircraft and other cylinder failures. For example, the National Transportation Safety Board (xe2x80x9cNTSBxe2x80x9d)-Materials Laboratory found that a cylinder barrel with 188.1 hours of service taken from a Skyhawk 172 Cessna aircraft which made an emergency landing in Independence, OR on Jun. 14, 1998 was cracked in fatigue for almost the entire circumference initiating from the outside between the fourth and fifth cooling fin roots. They observed multiple fatigue cracking initiated from pre-existing hairline cracks less than 0.001 in. in depth. No other material abnormalities were found. Textron Lycoming also found specks or hairline cracks ( less than 0.001 in. in depth) in the remaining cylinder barrels from the same aircraft engine. See Epperson, NTSB-Materials Laboratory Report No. 98-149 and -149A, September 1998; Kim, Textron Lycoming Materials Laboratory Report No. 11271, July 1998.
Subsequent follow-up investigation have revealed that specks or hairline cracks most likely were induced by CSCC during treatment of the cylinder barrel with a caustic black oxide solution during the manufacturing process. The black oxide bath currently typically used in the industry is composed of a solution containing 80% sodium hydroxide (NaOH), 10% sodium nitrate (NaNO3), and 10% sodium nitrite (NaNO2). Engine cylinder barrels coated with black oxide have an improved ability to retain oil on their surface, which in turn improves their scuff resistance or break-in, static color appearance, and minor corrosion resistance.
The observation that specks up to 0.00003 in. in depth were seen even in the first cylinder barrel machined, after a complete new set of tool bits was installed, and black oxided indicates the extent of the specks or hairline cracks observed may depend on the amount of residual stresses from machining that are present in the cylinder barrel. Specks or hairline cracks were seen on the cylinder barrel cooling fin roots where varying amounts of residual machining stresses were present. No specks or hairline cracks were observed prior to treatment with black oxide solution even though some surface irregularities begin to occur after the same tool bits are used to machine many cylinder barrels. Although the severity of observed specks or hairline cracks observed was sporadic in cylinder barrels numbered ninety through one hundred that had been machined with the same set of tool bits, more pronounced specks or hairline cracks were observed in machined and 80/10/10 black oxided cylinder barrels numbered 101 and thereafter.
The fact that the specks or hairline cracks were present only after black oxide treatment strongly suggests the cracking phenomenon is related to the caustic embrittlement of steel. It is possible that two basic reactions between hydrogen and steel are responsible for the mechanism of CSCC. First, during a corrosion reaction of steel from the aqueous phase, hydrogen adatoms (xe2x80x9cHadsxe2x80x9d) form on the steel""s surface according to the reactions:
Fe=Fe+++2exe2x88x92xe2x80x83xe2x80x83(1)
H++exe2x88x92=Hadsxe2x80x83xe2x80x83(2)
Hads produced via Equation (2) either may combine to form hydrogen molecules that evolve as gas bubbles or may become absorbed into the steel surface. Absorbed Hads diffuse into the steel to areas of high triaxial tensile stress, and embrittle the metal which eventually leads to the steel""s premature failure. Although surface modifications may be designed to decrease the rate of absorbance and increase the rate of evolution, whether Hads are absorbed or evolved depends on the energetics of the steel""s surface.
In the second possible reaction, molecular hydrogen is evolved on the steel surface during a corrosion reaction of steel from an aqueous phase. Pure molecular hydrogen gas then dissociates into atomic hydrogen (H) at clean deformed surfaces on the steel, according to the reactions:
2H++2exe2x88x92=H2xe2x80x83xe2x80x83(3)
H2=H+Hxe2x80x83xe2x80x83(4)
The dissociated atomic hydrogen migrates to regions of high triaxial tensile stress in the steel matrix.
During treatment with black oxide, magnetite (Fe3O4) is formed on the cylinder barrel surface according to the following reactions. NaOH reacts with water and iron to produce sodium iron hydroxide and molecular hydrogen under the reaction:
4NaOH+2H2O+Fe=Na4Fe(OH)6+H2xe2x80x83xe2x80x83(5)
The molecular hydrogen produced can either evolve into the air as gas bubbles or be absorbed into the steel surface as described above. The sodium iron hydroxide is then oxidized to form NaOH, water, and magnetite film on the steel surface according to the reaction:
3Na4Fe(OH)6+1/2O2=Fe3O4+12NaOH+H2Oxe2x80x83xe2x80x83(6)
The amount of atomic hydrogen diffusing into the steel surface, and thereby the likelihood that CSCC will occur, may be reduced if a sufficient amount of a reactant is available to react with the molecular hydrogen generated in Equation (5). For example, should a sufficient amount of sodium nitrite be available on the steel surface, molecular hydrogen may be removed according to the reaction:
2NaNO2+3H2=2H2O+N2+2NaOHxe2x80x83xe2x80x83(7)
Current conditions for black oxide treatment of cylinder barrels (80% NaOH, 10% NaNO3, 10% NaNO2) do not utilize reactants at concentrations sufficient to remove significant quantities of either molecular or atomic hydrogen from the black oxide bath, particularly since sodium nitrate may not remove hydrogen from the metal surface. It is therefore desirable to provide a process for oxide treatment of aircraft engine cylinder barrels which can remove a significant amount of molecular and/or atomic hydrogen during the black oxide treatment process and thereby reduce the occurrence of CSCC. Further, because it appears that residual stresses from the machining process determine the extent of the specks or hairline cracks that will develop for the given black oxide bath, it is desirable to establish criteria to determine the maximum amount of cylinder barrels that can safely be machined from a given set of tool bits before replacement of these tool bits becomes obligatory. The present invention addresses this problem.
Accordingly, it is a general object of the present invention to provide a method of manufacturing cylinder barrels to be used in aircraft engines which will prevent the barrels"" premature cracking and failure due to caustic stress-corrosion cracking. More specifically, it is an object of the present invention to provide a method of manufacturing cylinder barrels to be used in aircraft engines wherein an improved black oxide treatment process substantially reduces the amount of specks and completely eliminates hairline cracks in the barrels, thereby preventing the barrels"" premature cracking and failure. It is a further object of the present invention to provide a method of manufacturing cylinder barrels to be used in aircraft engines wherein the contribution of residual stresses produced by machining to the barrels"" premature cracking and failure due to caustic stress-corrosion cracking is minimized.
An improved method of manufacturing aircraft engine cylinder barrels to minimize caustic stress corrosion cracking is provided wherein the improvement comprises the step of immersing a machined cylinder barrel in a black oxide chemical bath, wherein the bath comprises a solution comprised of about 60% sodium hydroxide, about 0% sodium nitrate, and about 40% sodium nitrite. In another embodiment, the process further includes machining no more than a selected number of cylinder barrels on a given set of tool bits, the number selected such that a surface of the barrels is essentially free of detectible specks or hairline cracks after black oxide treatment. The invention further includes aircraft engine cylinder barrels manufactured according to these improved processes wherein a surface of the machined engine cylinder barrel is essentially free of specks or hairline cracks after black oxide treatment.